The direct active imperative is obtained by removing the imperfect prefix from the required jussive/imperative stem.
Word / formMeaning / explanation
تَبِيعُ → تَبِعْ → بِعْsell! (one man).
تَبِيعَانِ → تَبِيعَا → بِيعَاsell! (two people).
تَبِيعُونَ → تَبِيعُوا → بِيعُواsell! (a group of men).
The masculine singular contracts to a short vowel, but dual and masculine-plural forms retain long ī. Therefore “بِيعْ” is not the correct singular masculine command.